A transformer tap is a connection point along a transformer winding that allows a certain number of turns to be selected. This means, a transformer with a variable turns ratio is produced, enabling voltage regulation of the output. The tap selection is made via a tap changer mechanism. The tap changer is a mechanical device which obtains its power from a spring loaded actuator. The spring is loaded (charged) with energy to then be released (discharged) to provide a fast movement of a diverter switch which moves between different contacts, where diverter resistors are used to mitigate the transition state when the switch moves between different contacts when the tap changer is on-load (without halting the operation of the transformer). A diverter is a resistor used to divert part of an electric current, as one connected in shunt with the series winding or with the commutating-pole winding of a machine.
GB 980,677 discloses a mechanism capable of producing repeated rotation of an output shaft in alternate senses between two latching positions as a result of continued rotation of an input shaft, including resilient driving means connected to drive the output shaft, charging means for said resilient driving means and operated by the input shaft, first and second latching means for holding the output shaft in said two latching positions, and means operated by the input shaft for releasing first said latching means when the shaft is in one said latching position and the resilient driving means is charged, the arrangement being such that the resilient driving means then drives the output shaft to the other latching position, where it engages with second said latching means. One embodiment of the document discloses a mechanism for driving an output shaft, which in turn operates the diverter switches of an on-load transformer tap-changer.
U.S. Pat. No. 6,347,615 discloses a tap-changer vacuum switch having an actuating rod which is extending and displaceable along an axis and which is provided with a damper having a damper housing offset from and fixed relative to the vacuum switch, and a rod piston fixed on the valve-actuating rod, in the damper housing. The damper housing is formed with a pair of radially open ports opening into the compartment. An in-only check valve fitted to one of the ports only permits fluid flow into the compartment, and an out-only check valve fitted to the other port only permits fluid flow out of the compartment. The opening and closing pressures for these valves are largely determined by the constants of their springs. Since the spring constant is much less susceptible to change as its temperature changes, this means that the valves will perform uniformly whether hot or cold. The document is concerned with preventing bouncing of a vacuum switch by applying a constant pressure independent of the oil viscosity.
The time it takes the diverter switch to disconnect from a contact and to connect to another contact is important to achieve good operation of the tap changer. However, the velocity of the switch is dependent on the viscosity of transformer oil in which the tap changer operates, which viscosity is dependent on the temperature of the oil which can vary greatly over an operation cycle of the transformer. The velocity, and thus the switching time, is also dependent on the power of the actuator spring and on mechanical friction within the tap changer, parameters which may also vary over time du to general wear of the tap changer.